1. Field of the Invention
The present invention relates generally to a radio wave absorbing panel used for absorbing radio waves incident on a building or radio waves produced by OA equipment in an office and, more particularly, to a radio wave absorbing panel which both absorbs incident radio waves or OA-equipment-generated radio waves and transmits visible light.
2. Description of the Related Art
In recent years, along with increases in numbers of high-rise buildings, cases of radio waves of TV frequency bands such as VHF and UHF being reflected by buildings have become more common. Consequently, ghosting, which arises on a TV screen when radio waves arriving at the antenna of a TV receiver directly from a TV station (direct waves) and radio waves reflected by buildings (reflected waves) are incident on the antenna simultaneously, has become a serious social problem.
Use of wireless LANs (local area networks) in offices is rapidly spreading. Popularization of such wireless LANs has led to the expansion of the frequency band of radio waves used in offices to 1-6 GHz. It is thus expected that in-office radio wave environments will become deteriorated due to interferences by electromagnetic noises produced by various closely positioned OA equipment. This makes it necessary to reduce undesired radio waves in offices by using building materials provided in those offices.
For these reasons, with the object of reducing the number of the radio waves reflected by exterior walls of buildings or the radio waves generated in offices, radio wave absorbing panels made of magnetic materials such as ferrite have been affixed to or embedded in exterior walls or inside building materials to absorb those radio waves.
For window glass installed in building window openings, to reduce air-conditioning cooling loads in summer (to save energy), glass coated with a film having a heat ray reflecting function has been used; however, because films having a high heat ray reflecting capability have low electrical resistance, their reflectivity of radio waves is high and they are a cause of radio wave obstruction.
Radio wave absorbing panels for reducing radio wave reflection in which ferrite is used cannot be applied to window openings of buildings because ferrite does not transmit visible light. Consequently the situation has been that heat ray reflecting ability has been sacrificed and transparent heat ray reflecting films having relatively high electrical resistance have been coated on windows of buildings to reduce radio wave reflection and prevent radio wave obstruction by transmitting radio waves through window openings into buildings.
Concerning window glass for buildings and vehicles, technology is known (from, for example, Japanese Patent Laid-Open Publication Nos. HEI 3-250797, HEI 5-042623, HEI 5-050548 and HEI 7-242441) whereby, with the object of preventing obstruction due to radio wave reflection of high-performance heat ray reflecting films, high heat ray reflectivity and low radio wave reflectivity are realized at the same time by a conducting film being divided up into areas of a size amply smaller than the wavelength of incident radio waves to raise its radio wave transmittivity.
Radio wave absorbing panels in the related art which have radio wave transmittivity are an attempt to prevent radio wave reflection problems by providing window glass of buildings with radio wave transmittivity; however, associated with these there are the problems that incoming radio waves penetrate to the inside of the building and affect office equipment such as personal computers and that electromagnetic waves radiated from electronic appliances inside the building leak through the window glass to outside the building. Although this kind of radio wave obstruction is expected to increase in the future, no effective countermeasure has been taken besides reflecting and thereby blocking radio waves by using a conducting wire mesh or a conducting film on windows of buildings, and in districts where there is a likelihood of a radio wave reflection obstruction arising it has been difficult to build buildings which have large-area windows and block radio waves.
To solve this problem, it is necessary to create a practically usable transparent panel which absorbs radio waves instead of reflecting or transmitting them.
At present there are radio wave absorbing panels made by disposing in parallel two transparent substrates each coated with a conducting film having a controlled sheet resistivity, with which panels it is possible to realize a high radio wave absorbing capacity by utilizing resonance caused by interference of multiple reflections of radio waves. However, to absorb VHF band (about 100 MHz) radio waves, the gap between the two substrates constituting the radio wave absorbing panel must be made from several tens of cm to over a meter, and therefore such panels cannot be realistically applied to ordinary windows of buildings.
When a wireless LAN uses a frequency of 2.4 GHz, the two substrates need to be spaced about 31 mm apart. This makes the substrates inconvenient for use as building materials.
It is therefore an object of the present invention to provide a radio wave absorbing panel having superior radio wave absorbency and transparency to light.
To achieve this and other objects, the invention provides a radio wave absorbing panel comprising at least two insulating substrates disposed in parallel a predetermined distance apart on at least one side of each of which is formed a continuous conducting film and at least one insulating substrate disposed between these insulating substrates in parallel therewith on a surface of which are formed conducting films disposed in the form of stripes or in the form of a matrix.
As a result, in the invention, even when the relative permittivity of the radio wave absorbing panel is made large and the thickness of the panel is made thin, it is possible to make its absorptance of radio waves high, and consequently the invention can be applied for example to a panel for absorbing radio waves of VHF band and LAN frequency band highly suitable for installation in a window opening of a building.
In a radio wave absorbing panel according to the invention, when stripe-form conducting films are used, preferably, the sheet resistivity of the continuous conducting film is from 1xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 and when the width and the sheet resistivity of each of the conducting films disposed in the form of stripes are respectively written Hcm and RBMxcexa9/xe2x96xa1 and the insulation resistance of the insulating substrate on which the stripe-form conducting films are formed is written RDcmxcexa9, then H, RBM and RD are set in the ranges of: 0.1 cmxe2x89xa6Hxe2x89xa650 cm, 1xcexa9/xe2x96xa1xe2x89xa6RBMxe2x89xa640xcexa9/xe2x96xa1, RDxe2x89xa630,000 cmxcexa9. when stripe-form conducting films having these values are used, the relative permittivity of the radio wave absorbing panel can be made large and its absorptance of radio waves raised, and even if the panel is made thin it is possible to improve its absorbency of radio waves arriving from a fixed direction.
When matrix-form conducting films are used, preferably, the sheet resistivity of the continuous conducting film is from 1xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 and when the width, the length and the sheet resistivity of each of the conducting films disposed in the form of a matrix are respectively written Hcm, Vcm and RBMxcexa9/xe2x96xa1 and the insulation resistance of the insulating substrate on which the matrix-form conducting films are formed is written RDcmxcexa9, then H, V, RBM and RD are set in the ranges of: 0.1 cmxe2x89xa6Hxe2x89xa650 cm, 0.1 cmxe2x89xa6Vxe2x89xa650 cm, 1xcexa9/xe2x96xa1xe2x89xa6RBMxe2x89xa640xcexa9/xe2x96xa1. When matrix-form conducting films having these values are used, the relative permittivity of the radio wave absorbing panel can be made large and its absorptance of radio waves raised, and even if the panel is made thin it is possible to improve its absorbency of radio waves arriving from any direction.
Also, when stripe-form conducting films are used, preferably, the sheet resistivity of the continuous conducting film formed on the surface of one of the insulating substrates is not more than 30xcexa9/xe2x96xa1 and the sheet resistivity of the continuous conducting film formed on the surface of another of the insulating substrates is from 50xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 and when the width and the sheet resistivity of each of the conducting films disposed in the form of stripes are respectively written Hcm and RBMxcexa9/xe2x96xa1 and the insulation resistance of the insulating substrate on which the conducting films disposed in the form of stripes are formed is written RDcmxcexa9, then H, RBM and RD are made: 0.1 cmxe2x89xa6Hxe2x89xa650 cm, 1xcexa9/xe2x96xa1xe2x89xa6RBMxe2x89xa640xcexa9/xe2x96xa1, RDxe2x89xa730,000 cmxcexa9. By making the sheet resistivity of the conducting film on one of the insulating substrates not more than 30xcexa9/xe2x96xa1 and the sheet resistivity of the conducting film of another of the insulating substrates from 50xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 in this way, it is possible to make the panel still thinner while maintaining an ample radio wave absorbency.
And when matrix-form conducting films are used, preferably, the sheet resistivity of the continuous conducting film formed on the surface of one of the insulating substrates is not more than 30xcexa9/xe2x96xa1 and the sheet resistivity of the continuous conducting film formed on the surface of another of the insulating substrates is from 50xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 and when the width, the length and the sheet resistivity of each of the conducting films disposed in the form of a matrix are respectively written Hcm, Vcm and RBMxcexa9/xe2x96xa1 and the insulation resistance of the insulating substrate on which the conducting films disposed in the form of a matrix are formed is written RDcmxcexa9, then H, V, RBM and RD are set to: 0.1 cmxe2x89xa6Hxe2x89xa650 cm, 0.1 cmxe2x89xa6Vxe2x89xa650 cm, 1xcexa9/xe2x96xa1xe2x89xa6RBMxe2x89xa640xcexa9/xe2x96xa1, RDxe2x89xa730,000 cmxcexa9. By making the sheet resistivity of the conducting film on one of the insulating substrates not more than 30xcexa9/xe2x96xa1 and the sheet resistivity of the conducting film of another of the insulating substrates from 50xcexa9/xe2x96xa1 to 3000xcexa9/xe2x96xa1 in this way, and because matrix-form conducting films set to predetermined values are being used, it is possible to make the relative permittivity of the radio wave absorbing panel large and raise its radio wave absorptance.
Preferably, transparent plate glass is used as the insulating substrates and the stripe or matrix-form conducting films are transparent conducting films composed mainly of SnO2 or In2O3 or are metal films composed mainly of Ag, Au, Cu or Al, whereby it is possible to lower the sheet resistivity of the conducting films and raise their transmittivity of light.
Also, dry air may be preferably sealed in spaces between the at least two insulating substrates and the at least one insulating substrate on the surface of which are formed conducting films disposed in the form of stripes or a matrix, whereby condensation due to changes in outside temperature can be prevented and deterioration in radio wave absorbing capacity due to water in the conducting films can be prevented.
Alternatively, resin may be preferably sealed in spaces between the at least two insulating substrates and the insulating substrate on the surface of which are formed conducting films disposed in the form of stripes or a matrix to form a laminated glass structure, whereby it is possible to prevent the glass from cracking and scattering.